A CMOS imager circuit includes a focal plane array of pixel cells, each cell includes a photosensor, for example, a photogate, photoconductor or a photodiode overlying a substrate for producing a photo-generated charge in a doped region of the substrate. A readout circuit is provided for each pixel cell and includes at least a source follower transistor and a row select transistor for coupling the source follower transistor to a column output line. The pixel cell also typically has a floating diffusion node, connected to the gate of the source follower transistor. Charge generated by the photosensor is sent to the floating diffusion region. The imager may also include a transistor for transferring charge from the photosensor to the floating diffusion node and another transistor for resetting the floating diffusion region node to a predetermined charge level prior to charge transference.
In a CMOS imager, the active elements of a pixel cell, for example a four transistor pixel, perform the necessary functions of (1) photon to charge conversion; (2) transfer of charge to the floating diffusion node; (3) resetting the floating diffusion node to a known state before the transfer of charge to it; (4) selection of a pixel cell for readout; and (5) output and amplification of a signal representing a reset voltage and a pixel signal voltage based on the photo converted charges. The charge at the floating diffusion node is converted to a pixel output voltage by a source follower output transistor.
FIG. 1 illustrates a block diagram of a CMOS imager device 308 having a pixel array 240 with each pixel cell being constructed as described above. Pixel array 240 comprises a plurality of pixels arranged in a predetermined number of columns and rows. The pixels of each row in array 240 are all turned on at the same time by a row select line, and the pixels of each column are selectively output by respective column select lines. A plurality of rows and column lines are provided for the entire array 240. The row lines are selectively activated by the row driver 245 in response to row address decoder 255 and the column select lines are selectively activated by the column driver 260 in response to column address decoder 270. Thus, a row and column address is provided for each pixel.
The CMOS imager is operated by the control circuit 250 which controls address decoders 255, 270 for selecting the appropriate row and column lines for pixel readout, and row and column driver circuitry 245, 260 which apply driving voltage to the drive transistors of the selected row and column lines. The pixel column signals, which typically include a pixel reset signal Vrst and a pixel image signal Vsig for each pixel are read by sample and hold circuitry 261, 262 associated with the column device 260. A differential signal Vrst-Vsig is produced for each pixel which is amplified and digitized by analog-to-digital converter 275. The analog to digital converter 275 converts the analog pixel signals received from the column driver 260 in its associated sample/hold circuits 261, 262 to digital signals which are fed to an image processor 280 to form a digital image.
Exemplary CMOS imaging circuits, processing steps thereof, and detailed descriptions of the functions of various CMOS elements of an imaging circuit are described, for example, in U.S. Pat. No. 6,140,630 to Rhodes, U.S. Pat. No. 6,376,868 to Rhodes, U.S. Pat. No. 6,310,366 to Rhodes et al., U.S. Pat. No. 6,326,652 to Rhodes, U.S. Pat. No. 6,204,524 to Rhodes, and U.S. Pat. No. 6,333,205 to Rhodes. The disclosures of each of the forgoing are hereby incorporated by reference herein in their entirety.
A schematic diagram of an exemplary CMOS pixel four-transistor (4T) pixel cell 10 is illustrated in FIG. 2. The four transistors include a reset transistor 32, a source follower transistor 34, a row select transistor 36 and a transfer gate 30. A photosensor 26 converts incident light into a charge. A floating diffusion region 28 receives charge from the photosensor 26 through the transfer gate 30 and is connected to the reset transistor 32 and the source follower transistor 34. The source follower transistor 34 outputs a signal proportional to the charge accumulated in the floating diffusion region 28 to a sampling circuit when the row select transistor 36 is turned on. The reset transistor 32 resets the floating diffusion region 28 to a known potential prior to transfer of charge from the photosensor 26. The photosensor 26 may be a photodiode, a photogate, or a photoconductor. If a photodiode is employed, the photodiode may be formed below a surface of the substrate and may be a buried p-n-p photodiode, buried n-p-n photodiode, a buried p-n photodiode, or a buried n-p photodiode, among others.
Image sensors, such as an image sensor employing the conventional pixel cell 10, have a characteristic light dynamic range. Light dynamic range refers to the range of incident light that can be accommodated by an image sensor in a single frame of pixel data. It is desirable to have an image sensor with a high light dynamic range to image scenes that generate high light dynamic range incident signals, such as indoor rooms with windows to the outside, outdoor scenes with mixed shadows and bright sunshine, night-time scenes combining artificial lighting and shadows, and many others.
The electrical dynamic range for an image sensor is commonly defined as the ratio of its largest non-saturating signal to the standard deviation of the noise under dark conditions. The electrical dynamic range is limited on an upper end by the charge saturation level of the sensor, and on a lower end by noise imposed limitations and/or quantization limits of the analog to digital converter used to produce the digital image. When the light dynamic range of an image sensor is too small to accommodate the variations in light intensities of the imaged scene, e.g. by having a low light saturation level, the full range of the image scene is not reproduced. The illumination-voltage profile of the conventional pixel 10 is typically linear as shown in FIG. 40A, which illustrates an illumination v. voltage graph of a prior art pixel cell. A pixel cell's maximum voltage Vmax may be reached at a relatively low level of illumination Imax which causes the pixel cell to be easily saturated, thus limiting the dynamic range of the pixel. The relationship between electrical dynamic range and light dynamic range is shown in FIGS. 40A and 40B.
When the incident light captured and converted into a charge by the photosensor during an integration period is greater than the capacity of the photosensor, excess charge may overflow and be transferred to adjacent pixels. This undesirable phenomenon is known as blooming and results in a bright spot in the output image. Thus, there is a desire and need for a pixel cell having improved saturation response and lower potential for blooming.